Electrostatic interactions induced formation of sandwich nanostructures and their usage as SERS substrates

Metal nanoparticles have gained significant attention over the past few decades due to their interesting physico-chemical properties, leading to a myriad of applications in catalysis, sensing, biomedicine, etc. More recently, metal nanostructures have gained much interest for surface enhanced Raman scattering (SERS) sensing of various bio(analytes) of interest, wherein an increasing focus on improving the detection limit of analytes has spurred significant activity towards development of new SERS substrates. In this work, we demonstrate for the first time, the formation of a `metal-analyte-metal¿ sandwich structure, which significantly improves the detection limit of the analyte (rhodamine dye) under investigation. Specifically, gold nanospikes were electrodeposited on a substrate as shown by us previously (Chem Commun. 2009, 5039), and functionalized with dye molecules using electrostatic interactions. To obtain `metal-analyte-metal¿ sandwich structures, the dye-functionalized gold nanospike substrates were drop-casted with gold, silver, and gold-silver alloy nanoparticles synthesized separately by amino acids-mediated reduction mechanism. The controlled positioning of the analyte dye molecules between gold nanospikes and metallic/ bimetallic nanoparticles leads to an increase in their Raman scattering cross sections thus significantly improving the SERS detection capability of analyte. The influence of the surface charge of metal nanoparticles and metallic composition on SERS signal enhancement was also investigated, which provided insight about the mechanism of signal enhancement.